What is the fastest and easiest way to add a vignette in the compositer for Cycles?
I usually do something like this:

It doesn't take too long to setup (three nodes), and it's provides a high degree of control:

The aspect (or even rotation) can be controlled via the settings of the Ellipse Mask* node.
The softness can be controlled with the Blur node
The amount of influence can be controlled via the mix node factor.

For a more automated solution
You could try something like this (click for full size):

This can be grouped into a single node (and included in your startup file) with very simple controls: